Examples from prior art in the field of well orientation measurement are described in U.S. Pat. Nos. 4,435,454; 4,472,884; 4,559,713; 4,819,336.
While drilling wells and notably more or less deviated deep wells, the angle of inclination of the well and the azimuth thereof are generally to be determined with precision. To that effect, a measuring device included in a drill string inserted above the drilling tool is used.
This device generally comprises three magnetometers for measuring the components of the local magnetic field vector in three orthogonal directions Ox, Oy, Oz. One of the axes Oz is parallel to the axis of the tool and of the drill string. The other two, Ox, Oy are in a plane orthogonal to the axis of the drill string and the orientation thereof with respect to the vertical can be any orientation. Three accelerometers for determining the components Gx, Gy, Gz of the local gravitation vector are also arranged along these three axes. The measurements of the accelerometers enable a calculation of the inclination I of the tool and the orientation thereof, often referred to as TF (for Tool Face), which is the angle between axis Ox and the vertical plane. By combining the measurements Bx, By, Bz of the three magnetometers with the values I and TF obtained, the azimuth of the tool and therefore of the well, which is the angle between the projections in the horizontal plane of the axis of the tool and of the magnetic field, can be calculated.
The drill string, which is made of metal, magnetizes under the influence of the terrestrial magnetic field. Thus, the drill string generates a parasitical magnetic field which is superposed on the terrestrial field and alters the measurements. In order to minimize the parasitical influence, the measuring tool is interposed in a certain length of drill collar made of a nonmagnetic material. The residual disturbance P due to the more distant magnetic parts of the drill string is then assumed to be parallel to the axis of the drill string (FIG. 5).
In fact, the existence of a local magnetization ("hot spots") of pipes reputed to be nonmagnetic is often observed. The field generated by these anomalies is generally not parallel to the axis of the drill string. The case of a magnetic disturbance P (FIG. 6) of any direction provided with an axial component (axial disturbance) along Oz but also a radial component (radial disturbance) orthogonal to the previous one is thus to be considered.
U.S. Pat. No. 4,163,324 describes a method for eliminating errors due to a magnetic disturbance in the case where the latter may be assumed to be purely axial.
In the case, justified in practice, where no hypothesis can be made on the direction of the disturbance field, a method described in U.S. Pat. No. 4,682,421 may be used, which mainly consists in eliminating the influence thereof by rotating the measuring device about the axis thereof which is substantially parallel to the local direction of extension of the well and, for different angular positions distributed over 360.degree., in measuring the components of the magnetic field vector. The transverse component of the magnetic disturbance can be eliminated after comparing the measurements performed in several different orientations.
When the drilling tool is connected to a surface maneuvering installation by a rigid drill string which is progressively extended by fixing pipe sections, the measuring method mentioned above may be implemented, for example when the string is being extended. Progression of the tool is stopped. The string is turned around on itself and the measuring devices are rotated thereby. The successive positions thereof are distributed in a circle in a plane transverse to the direction of extension of the well. Measurements are repeated for different successive angular positions in the same longitudinal place in the well.
Each measuring sequence is relatively long, of the order of ten minutes, for example. The multiplicity of the measurements to be performed in each stop place causes an undoubted slowing down of the drilling rate if each sequence is repeated at regular intervals. Stopping of the tool presents another drawback in the relatively frequent case where turbodrilling is performed. The tool is driven by a bottomhole turbine brought in rotation by a mud flow circulating in the drill string and in the annulus between the string and the well. The rotation of the measuring tool linked to the drilling tool, from one angular position to the next one, requires maintaining a mud flow which tends to enlarge the well and to cause instability zones therein.